The long term goal of this project is to study the development, maintenance, and migration mechanisms of T cells that help B cells, namely T follicular helper cells (Tfh), in immunity and autoimmunity. -Specific Aim 1. Assess the signals necessary for the development of Tfh cells: These signals will be investigated by setting up a model system to polarize CD4 T cells in vitro using a combination of factors hypothesized to be important for their development, such as IL-21, IL-4, and ICOS. Tfh cell polarization will be assessed by characterization of surface markers and cytokine production. In addition, we will use retroviral overexpression to determine if Bcl-6, a candidate lineage specific transcription factor for Tfh cells, is sufficient for their development. -Specific Aim 2. Investigate the requirement for ICOS signaling in Tfh maintenance in chronic autoimmunity, and determine the antigen-presenting cells necessary for provision of the ICOS signal via its ligand, B7RP-1 (ICOSL): GC's and Tfh cell numbers will be evaluated after blocking ICOS signaling well after immunization of normal mice as well as in autoimmune murine models at time points well after disease onset where GC's and Tfh cells are known to be present. The cells involved in providing ICOSL signals will be investigated using mice models where B cells or dendritic cells are made conditionally deficient in ICOSL. Flow cytometry and microscopy will be used to determine the development of Tfh cells and GC's following immunization. -Specific Aim 3. Analyze the trafficking mechanisms of Tfh cells into B cell follicles. The function of P-selectin glycoprotein ligand 1 (PSGL-1) as a homing and retention signal for T cells in the T zone will be investigated using flow cytometry and microscopy after immunization of a variety of genetically altered mice including PSGL-1 deficient and PSGL-1 transgenic mice. Tfh cells are known as B cell helpers of differentiation, proliferation, and isotype switching in T-dependent immune responses. Characterizing their development, maintenance, and trafficking mechanisms may provide new clues for treatment targets in immune dysregulation such as chronic systemic autoimmunity.